Derby Talk

Derby Talk is a forum for Pinewood Derby, Awana Grand Prix, Kub Kar Rally, Shape N Race Derby, Space Derby, Raingutter Regatta and other similar races where a child and an adult work together to create a race vehicle and a lot of fun and memories

So, a recent discussion has prompted some research into light sensitivity of Pinewood Derby timers. Namely, how most timers are affected by large amounts of ambient light. Sunlight, camera flashes, etc, seem to have the potential to oversaturate the light sensors and prevent them from detecting when a car has crossed the finish line.

I've had one person tell me they've even seen timers triggered by a camera flash. I have to admit that I don't understand the physics of how that would work, but if somebody knows, I'm listening!

Anyhow, I was thinking about how most (all?) finish line detectors use infrared emitters and sensors. Perhaps there's a better way?

Laser emitters are dirt cheap nowadays. I mean, I can buy a laser pointer at the dollar store. What if we built a finish line timer that used lasers as the light source, and some kind of photo sensor that was set to require a particularly high intensity of incoming light (e.g. from a laser beam) to register?

As an added bonus, because the beam to a laser can be precisely directed, the sensor could be recessed down a small and narrow space (a tube, perhaps), to further cut down on any light pollution.

Any thoughts on this? Reasons it's a Bad Idea(tm)?

Bonus: It would also provide a cool effect as the cars ran under the laser and the beams diffracted off of them.

I think part of the appeal of infrared or visible light is that it doesn't have to be *precisely* aligned, just aligned enough to get enough light in the hole to keep the phototransistor activated.

With the laser approach, you would need to keep precise alignment throughout the race. I'm not saying it couldn't be done, but it would definitely require more care during construction, and be easier to get misaligned during transport/storage etc. With a sufficiently rigid frame and support, it could definitely work, but when you think about the labor to build a rigid support structure and align 3-8 lasers on most tracks, it's definitely going to add to the cost significantly (either money or time). My MicroWizard finish line has the infrared LEDs simply attached to the display boards, and they can easily be bend one way or another for minor alignment changes. With lasers you'd need to put in adjustment screws or something else with good precision. For most folks I think it is a "not broke, not going to fix" thing. Most races are indoors, and the use of infrared sensors helps to keep camera flashes from affecting things (at least in my experience).

With regard to the person who says a camera flash triggered a finish, it is possible that the finish line in question was constructed using the sometimes suggested idea of putting the emitter and the receptor both below the track, and putting reflective material on the bottom of the front of the cars. The idea is that when the car passes over, it reflects the light back down, where it is detected. I've never seen an actual gate like this, but in construction discussions the idea seems to come up now and then. Such a setup might be able to interpret a camera flash as a finish. I can't think of any other situation where that would happen, unless the light frequencies between the emitter and the flash bulb were such, and the angles were just so, to create an interference pattern in the hole to the receptor which interrupted the signal long enough to detect a finish. Such an occurrence might be theoretically possible, but I somehow think it would be very very very very very unlikely.

Vitamin K wrote:Namely, how most timers are affected by large amounts of ambient light. Sunlight, camera flashes, etc, seem to have the potential to oversaturate the light sensors and prevent them from detecting when a car has crossed the finish line.

I always knew this was an issue but I was still amazed when I was helping to setup an outdoor track and even when putting my palm over the light sensor (phototransistor) on the track it was still 'seeing' light. Putting a leather wallet over the sensor finally let it 'see' dark. The solution was to build a foot long tunnel before and after the timer to block out the ambient light.

But it all comes down to the wavelength of the light used. I'm not a laser expert but a quick Google search indicates that most lasers are in the visible or infrared spectrum, which doesn't help with the ambient light factor.

Vitamin K wrote:As an added bonus, because the beam to a laser can be precisely directed, the sensor could be recessed down a small and narrow space (a tube, perhaps), to further cut down on any light pollution.

This would definitely help and utilizing a similar mount/tube/holder for normal sensors would be beneficial.

There are emitters and collectors that are tuned, so that ambient light has little to no effect. John Shreffler at http://www.newdirections.ws/judge.html uses these in his Judge timer. He also has a version that can be used in direct sunlight. Maybe John can chime in with some additional information.